After Chow Chi Yerk had drugged Cheung Mo Gei, Tse Tsun, and Chiu Mun on Snake Island, she had Chiu Mun completely at her mercy. At that point, Chow Chi Yerk could have just murdered Chiu Mun and because Cheung Mo Gei and Tse Tsun were in a deep sleep from the 10 Fragrances drug, nobody would have been the wiser (except Yan Lei, but Chow Chi Yerk would soon dispose of her). If Chiu Mun were to live, she would still be a source of trouble for Chow Chi Yerk later (as was the case). Wouldn't it have worked to Chow Chi Yerk's greater advantage if she'd just killed Chiu Mun then and there?